This invention relates to optical communications and, more specifically, to the handling of multiple optical communications channels in a wavelength division multiplexed communications system.
Many optical communications systems use wavelength division multiplexing to break up the bandwidth of a communications path into multiple channels. In recent years, such systems have been using narrower and narrower channel spacings. As a result, however, it is becoming more difficult to demultiplex channels with adequate isolation and low loss. For example, as the channel spacings decrease, the cross talk between adjacent channels tends to increase. As a result, demultiplexers with relatively narrow spacing are desirable, but difficult to fabricate.
An alternative to using a single narrow bandwidth demultiplexer to separate all the channels simultaneously is to use a two-step approach. In a first stage, a device is used to separate the channels into two groups, each group consisting of every other channel. That is, the channels are separated into xe2x80x9coddxe2x80x9d and xe2x80x9cevenxe2x80x9d channels. Two coarse multiplexers are then used to demultiplex the separated channels. In the past, the first stage devices that do the initial separation have been referred to as xe2x80x9cspectral slicersxe2x80x9d or xe2x80x9cinterleavers.xe2x80x9d Although fiber Bragg gratings provide excellent reflective properties for narrow channels, they have not been readily used as interleavers for demultiplexing many wavelengths. The reason is that a separate grating would be required for each channel. Moreover, to provide the necessary separation of odd and even channels, an optical circulator would have to be used with each grating. Optical circulators are relatively complex devices, and add greatly to the cost of a system.
Another limitation of conventional fiber Bragg gratings when it comes to their use in interleavers is the fact that the refractive index difference between different segments of the grating tends to be relatively low. This is due to the use of ultraviolet light exposure to fabricate the grating. By laterally exposing the core of the fiber to a periodic pattern of intense ultraviolet light, a permanent change in the refractive index of the fiber""s core is produced, creating a fixed index modulation according to the exposure pattern. However, this manner of forming the grating does not produce a particularly high refractive index contrast. A grating operates as a basic reflector stack, and the spectral width of such a reflector stack is proportional to the index contrast available, for a given reflectivity. This is because the spectral width is inversely proportional to the penetration depth of the light, and the higher the index contrast, the smaller the penetration. The bandwidth can be increased slightly by chirping the grating. However, if the chirp is too great, internal modes begin to appear within the grating.
When using a grating, the option of trading bandwidth for reflectivity also exists. By making the grating short, the bandwidth will be high, but the reflectivity will be low. However, for a square wave response interleaver, high reflectivity is required. Therefore, it is desirable to use a short grating, but one that has a high reflectivity. A low refractive index contrast in the grating is therefore detrimental to the interleaver design.
In accordance with the present invention, an optical demultiplexing apparatus is provided that separates a plurality of multiplexed optical channels in an input signal. Typically, the multiplexed wavelengths have an equal spacing in frequency. The input signal is directed to an optical interleaver that has a predetermined reflectivity response. That response is essentially a square wave in the frequency domain, such that the interleaver is highly reflective to each of a first set of channels while being highly transmissive to a second set of channels. The first and second channel sets are alternate channels along the wavelength band of the input signal. That is, the channels in the first and second set are interspersed in frequency in an alternating manner to make up the overall plurality of optical channels.
An optical coupler is provided that couples the reflected first set of channels from the interleaver to a first output path. Meanwhile, the transmitted channels of the second set are directed along a second output path. Coarse demultiplexers are used, respectively, to demultiplex the first set of channels and the second set of channels. Because of the separation of the alternate channel sets using the interleaver, the first and second channel sets each have significantly more wavelength spacing between them than in the original input signal. Thus, the demultiplexers are not as susceptible to cross talk as they might otherwise be.
The interleaver may take a number of different forms, for example, a waveguide diffraction grating. A diffraction grating is provided that has the desired reflectivity profile and therefore performs the interleaver function. In one embodiment, the grating is a sampled grating. That is, the grating has a structure with a constant pitch, but periodic breaks in the grating structure. The overall grating, however, produces the desired square wave response. The sampled grating may be apodized, and may be chirped. Indeed, whether or not the grating is a sampled grating, chirping or apodization may be used to improve the response. In another embodiment, the interleaver is a planar waveguide with a section having a periodic fluctuation in the refractive index conditions of the waveguide. For example, the change in refractive index conditions may be produced by a region of the core that is segmented such that sections of the core are replaced by material having a lower index of refraction than the core. Alternatively, a plurality of high refractive index material layers may be distributed in a periodic manner adjacent to a section of the core, such that they evanescently couple to the optical mode. Similarly, the cladding material may have a plurality of gaps distributed in a periodic manner adjacent to a section of the core, such that air or other ambient material borders the core in those gaps.
In the invention, a coupler is used to couple the reflected first set of channel wavelengths to an output path, rather than back to the input. In a first embodiment, the coupler is an optical circulator that conveys the input signal to the interleaver, while also conveying the channels reflected from the interleaver to a first output path. In another embodiment, the coupler makes use of a tilted grating type interleaver that redirects the reflected wavelengths to a first output path, while allowing transmitted wavelengths to be redirected to a second output path. The need for a lens for focusing into the first output path may be eliminated if the grating structure is curved. The curved grating may also function as the interleaver, given the proper system geometry.